Optical measurement device and measurement method

Abstract

An optical measurement device and an optical measurement method that can accurately measure a distance and a velocity by removing an influence of a nonlinear chirp of a laser in FMCW LiDAR. The optical measurement device includes a laser that outputs light of which a frequency is modulated; a photodetector; a heterodyne optical system that splits output light of the laser into two components, uses one as probe light, uses the other as reference light, adds a frequency shift to one of the probe light and the reference light, applies the probe light to an object, combines scattered light from the object and the reference light, and causes the combined light to be incident on the photodetector; an IQ detector; and an arithmetic operation processor.

The invention claimed is: 
     
       1. An optical measurement device comprising:
 a laser that outputs light of which a first frequency is modulated; 
 a photodetector; 
 a heterodyne optical system that splits output light of the laser into two components, uses one as probe light, uses the other as reference light, adds a frequency shift to one of the probe light and the reference light, applies the probe light to an object, combines scattered light from the object and the reference light, and causes the combined light to be incident on the photodetector; 
 an IQ demodulator that removes a component of the frequency shift, and outputs an in-phase component I and an orthogonal component Q by receiving a beat signal and an output of the heterodyne optical system as an input; and 
 an arithmetic operation processor that calculates a phase and a second frequency of the beat signal from the in-phase component I and the orthogonal component Q and excludes a component of a Doppler shift from the phase, calculates the average value of absolute values of one modulation period, and calculates the distance to the object on the basis of a proportional relationship between the distance and the average value of the absolute values of the phase which has been calibrated in advance. 
 
     
     
       2. The optical measurement device according to  claim 1 , wherein the arithmetic operation processor calculates the Doppler shift due to movement of the object and a velocity from an average value of one modulation period of the second frequency of the beat signal. 
     
     
       3. The optical measurement device according to  claim 1 , the optical measurement device further comprising a two-channel AD converter that acquires the in-phase component I and the orthogonal component Q detected by the IQ detector as a digital IQ signal,
 wherein the digital IQ signal is input to the arithmetic operation processor and the phase and the second frequency of the beat signal are calculated from the digital IQ signal. 
 
     
     
       4. The optical measurement device according to  claim 1 , the optical measurement device further comprising a one-channel AD converter that acquires the beat signal generated from the photodetector as a digital RF signal,
 wherein the IQ detector detects the in-phase component I and the orthogonal component Q from the digital RF signal, and 
 wherein the arithmetic operation processor calculates the phase and the second frequency of the beat signal from the in-phase component I and the orthogonal component Q. 
 
     
     
       5. The optical measurement device according to  claim 1 , wherein the frequency shift is greater than frequencies included in the in-phase component I and the orthogonal component Q. 
     
     
       6. The optical measurement device according to  claim 1 , wherein a frequency modulation signal of the laser is a sinusoidal wave. 
     
     
       7. An optical measurement method comprising:
 splitting output light of a laser that outputs light of which a first frequency is modulated into two components, using one as probe light, using the other as reference light, adding a frequency shift to one of the probe light and the reference light, applying the probe light to an object, combining scattered light from the object and the reference light, and causing the combined light to be incident on a photodetector; 
 removing a component of the frequency shift, and outputting an in-phase component I and an orthogonal component Q by receiving a beat signal and an output of the combined light as an input; 
 calculating a phase and a second frequency of the beat signal from the in-phase component I and the orthogonal component Q; excluding a component of a Doppler shift from the phase; 
 calculating the average value of absolute values of one modulation period; and 
 calculating the distance to the object on the basis of a proportional relationship between the distance and the average value of the absolute values of the phase which has been calibrated in advance. 
 
     
     
       8. The optical measurement method according to  claim 7 , wherein the arithmetic operation of calculating the velocity of the object includes calculating the Doppler shift due to movement of the object and a velocity from an average value of one modulation period of the second frequency of the beat signal. 
     
     
       9. The optical measurement method according to  claim 7 , wherein the in-phase component I and the orthogonal component Q are converted to a digital IQ signal and the phase and the second frequency of the beat signal are calculated from the digital IQ signal at the time of detecting the in-phase component I and the orthogonal component Q of the beat signal generated from the photodetector and calculating the phase and the second frequency of the beat signal from the in-phase component I and the orthogonal component Q. 
     
     
       10. The optical measurement method according to  claim 7 , wherein the beat signal is converted to a digital RF signal, the in-phase component I and the orthogonal component Q are detected from the digital RF signal, and the phase and the second frequency of the beat signal are calculated from the in-phase component I and the orthogonal component Q at the time of detecting the in-phase component I and the orthogonal component Q of the beat signal generated from the photodetector and calculating the phase and the second frequency of the beat signal from the in-phase component I and the orthogonal component Q.